This invention relates generally to the field of medical syringes used for injecting a medicinal liquid into a patient. More specifically, it relates to a syringe adapted for use with a disposable hypodermic needle and a compressible ampul that is either pre-filled with a medicinal liquid, or empty (or evacuated) for filling with a bodily fluid extracted from the patient.
Conventional hypodermic syringes comprise a tubular reservoir with a fitting at one end for the attachment of a hypodermic needle. The other end of the reservoir is provided with a plunger. Syringes of this type must be filled from a separate vial or ampul in the well-known manner of pushing the plunger all the way into the reservoir, inserting the needle into the vial, and pulling the plunger out until the reservoir is filled to the desired level, as indicated by volumetric markings on the reservoir wall. Before injection, a small amount of liquid must be expressed through the needle to assure that no air is injected.
Several disadvantages of these conventional syringes have been recognized. For example, the need to fill the syringe from a separate vial is cumbersome and time-consuming, as is the need precisely to measure the dosage and then express the liquid to remove air from the syringe. Also, the plunger mechanism can be awkward to manipulate in certain situations, and, in any case, requires at least some degree of dexterity. This can present a problem in certain applications, such as in the application of local anesthesia, where an injection must be made into a relatively inaccessible part of the body. Patients, such as diabetics, who must perform self-injections, and particularly those who are elderly, weak, or infirm, sometimes find it difficult to inject themselves with the plunger-type syringe.
The awkwardness of plunger-type syringes also makes them inconvenient to use to withdraw a bodily fluid from a patient, such as when a blood sample is taken from a vein. This awkwardness had led to the development of alternative devices for collecting blood samples. One of the most popular of such devices comprises an evacuated test tube sealed with a rupturable stopper. The tube is used in conjuntion with a holder which comprises a cylindrical body having an open end and a closed end fitted with a double-pointed hypodermic needle. The needle has an interior portion extending axially into the interior of the holder, and an exterior portion adapted for insertion into a vein. To use this type of device, the exterior point of the needle is inserted into the vein, and then the tube is inserted into the holder, stoppered-end first, whereby the interior point of the needle ruptures the stopper, thereby allowing blood to be drawn from the vein in response to the reduced pressure inside the evacuated tube.
While the evacuated type device is more convenient to use than a plunger-type syringe for taking blood samples and the like, it does suffer certain drawbacks. For example, the insertion of the tube into the holder so as to rupture the stopper must be done carefully to avoid excessive movement of the needle in the vein, which could cause injury to the vein. Also, there is a reduced degree of manual control by the practitioner over the rate of blood withdrawal. This is a result of a fixed pressure differential being applied to the vein once the stopper is ruptured. In some cases, such as in the veins of infants or in veins weakened by disease, the pressure differential applied to the vein could be excessive, with possible vein collapse or tissue withdrawal resulting. In addition, occasional problems have been noted with contamination of the blood sample with the lubricant used to provide an air-tight seal around the stopper, or with the material of the stopper itself. Finally, a pipette must be used to extract multiple specimens ("aliquots") from the tube, thereby providing additional opportunities for contamination of the collected sample.
Accordingly, attempts have been made to provide alternative syringe designs which overcome the disadvantages of the plunger mechanism. One approach has been to provide a syringe with a self-contained reservoir or ampul pre-filled with a precisely measured amount of medicament. The ampul has flexible walls, so that its contents can be discharged by compressing or squeezing it. Devices of this type are exemplified in the following U.S. patents:
______________________________________ Patent No. Patentee ______________________________________ 798,093 Dean 1,222,814 Storz 2,618,263 Lakso, et al. 3,099,264 Hubbard 3,114,369 Hall 4,013,073 Cunningham 4,475,906 Holzner ______________________________________
While the above-listed prior art devices do, in varying degrees, avoid the aforementioned problems associated with the plunger-type syringe, none has found widespread acceptance by medical practitioners, for a variety of reasons. For example, some of the prior art devices offer insufficient physical support for the needle during injection, making their use awkward at best. Others require a degree of dexterity in their use that, while possibly an improvement over the plunger-type syringe, still renders them ill-suited for self-injection, especially by the physically feeble. Another drawback in some prior art devices is their inability to be used with disposable needles. In such devices, the syringe and needle are manufactured as a disposable unit, a practice some may consider wasteful. Also, in devices of this type some mechanism must be included to prevent the flow of liquid from the reservoir into the needle until an injection is to be performed. Such a mechanism adds to the complexity, and thus the cost, of such devices.
Still another shortcoming in the prior art discussed above is the difficulty in injecting only a portion of the ampul's volume. Specifically, once the ampul is partially squeezed to discharge a portion of its contents, any relaxation of the pressure applied to the ampul may allow the ampul to return to its original shape, due to the resilience or "shape memory" of the ampul's material, or due to back pressure experienced through the needle. The result could be the introduction of air into the ampul, or, if the needle has been kept inserted in the patient's body, the aspiration of tissue into the syringe. Even if the ampul is constructed to minimize the risk of air or tissue aspiration, such syringes lack any convenient means for the practitioner to "meter" measured portions of the ampul's contents over a period of time, as is frequently done in administering a local or spinal anesthetic. Thus, in short, squeeze-actuated syringes of the prior art have been "one-shot" devices, in which the entire volume of the ampul is advantageously injected by a single, continuous squeeze of the syringe, rather than intermittently, over a period of time.
There has thus been a long-felt, but not entirely satisfied, need for a syringe that contains a pre-measured reservoir or ampul of medicament, which provides good physical support for the needle and which offers an injection mechanism that is easy to manipulate without much dexterity or strength. The need has also been felt for a syringe of this nature which can use disposable needles, and which, therefore, does not require means for blocking flow from the reservoir into a pre-attached needle. In addition, there has been a need for a syringe that can be easily adapted for use as an aspirating syringe to collect bodily fluids from a patient, especially venous blood samples. Moreover, there has been a further need for a syringe having the aforementioned characteristics, which also allows for convenient metering of the syringe contents intermittently over a period of time.